[OZAPRS] TVI

Sat Dec 13 19:15:22 EST 2003

Hi Hamish,

If you pad the Network Analyser's RF Out with a 20dB attenuator, and again
pad the RF In with another 20dB, the network analyser will barely see any
impedance mismatch at all.

The notch frequency is obvious though!  Interesting to take a look at the
frequency response swept from DC to 1GHz...  you can see the notches at
145,
435 and 725MHz.

Once you can cut a quarter wavelength of 75 Ohm coax accurately, try
placing
two quarter-wave stub-based notch filters on the line, separated by
exactly
quarter of a wavelength.  Attenuation is greater than 60dB!

I modelled this on Puff many years ago (~ 1993? ).  At that time, I made
the
3 pole version for installation in a next-door neighbour of another VK6
who
was causing massive TVI when his 2m and 70cm array was directed right at
the
neighbour's TV antenna, as he uplinked to UO-14, UO-22, KO-23 etc on 2m.
100W into a 22 element yagi, at a range of 4m, caused absolutelyt no TVI
after the filter was fitted.

Very interesting, and very satisfactory;  I find such a simple, yet
effective, solution to be quite elegant.

Glad it was of some use.

73 Chris vk6kch

-----Original Message-----
From: ozaprs-bounces at marconi.ics.mq.edu.au
[mailto:ozaprs-bounces at marconi.ics.mq.edu.au]On Behalf Of Hamish Moffatt
Sent: Saturday, 13 December 2003 2:07 PM
To: ozaprs at marconi.ics.mq.edu.au
Subject: Re: [OZAPRS] TVI

Thanks for the suggestions Chris and all.

I made up a notch filter as you suggested. (Unfortunately, nobody sells
things like T-pieces for the Belling Lee connectors used on TVs, so it's
a bit messy). It's fixed the digital receiver up completely - previously
all channels were affected, but now I can transmit 50W FM from my FT-847
without any affect on the receiver at all. Great!

It didn't seem to help the analog TV receiver elsewhere in the house.
However, I did change the transmitter from a Yaesu FT-411 to an ICOM
IC-02A (also a handheld) which seems to have helped. I'll investigate
the FM900/828 suggestions later, since the IC-02A doesn't seem that
great either. I have an 828 somewhere but without suitable crystals.

As to tuning filters, I have access to a network analyser which is ideal
for it, except that it's for 50 ohms and the results are quite confusing
when trying to analyse a 75 ohm network....!

Thanks,

Hamish

On Fri, Dec 12, 2003 at 11:37:48PM +0800, Chris Hill wrote:
> Hi Hamish,
>
> You could try inserting a 2m notch filter inline with your TV antenna.
>
> 1.  Insert a "T-piece" where the 75 Ohm coax enters the first active
device
> in your TV system (usually the back of the VCR)
>
> 2.  Plug a 42cm length of 75 Ohm coax into the "spare" port of the
T-piece,
> making sure the far end of the coax is open circuit.
>
>
> (The 42cm is based on a guestimate that the velocity factor of the coax
is
> 80%)
>
> This should give you approx 25dB to 30dB of rejection of your 2m
> transmissions, but have minimal impact on your TV signal.
>
> Fastest and cheapest way to try this, is to buy a 2m long "TV coax
extension
> cable" from the local supermarket for $5 or so.
>
> If you need more attenuation, you can make a 3 pole filter, which gives
> approx 60dB attenuation...  but you really need access to a spec an and
> tracking generator for that, in order to get the accuracy.
>
> Oh, and as a free bonus, it will also attenuate amateur 70cm
> transmissions...
>
>
> Please let me know how you get on with it.
>
>
>
> 73 Chris vk6kch
>
>
>
>
> -----Original Message-----
> From: ozaprs-bounces at marconi.ics.mq.edu.au
> [mailto:ozaprs-bounces at marconi.ics.mq.edu.au]On Behalf Of Hamish Moffatt
> Sent: Friday, 12 December 2003 9:57 PM
> To: OZAPRS
> Subject: [OZAPRS] TVI
>
>
> Hi all,
>
> I am having some problems with TVI from the IGATE station. (Stupidly)
> the 2m antenna is on top of the TV antenna. This has not been much of an
> issue because we have Foxtel but lately we have been using the antenna a
> bit more and the IGATE is even affecting digital TV reception to the
> digital TV tuner card in my PC. It looks like it is just overload; I
> thought it might be unclean signal (the radio is still a handheld)
> but I just tried transmitting fairly low power from the FT-847 and also
> had trouble.
>
> I have another antenna located a few metres away from the TV antenna
> but it has too much gain and even the FT-847 is badly affected by
> pagers. How resistent to pagers is the FM900? I have one which I'm
> not using and could put in, if it will cope with the pagers okay.
>
> If I get around to it I might test the current radio on a scope to see
> how clean the signal is - so it could be down for a bit.
> Else I might be able to move the low gain antenna over the Christmas
> break. If you have any suggestions they would be most welcome.
>
>
> Thanks,
> Hamish
> --
> Hamish Moffatt VK3SB <hamish at debian.org> <hamish at cloud.net.au>
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--
Hamish Moffatt VK3SB <hamish at debian.org> <hamish at cloud.net.au>
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